I used this deck to clear Journey in the Dark twice consecutively. It is an evolution of my deck from the previous scenario, but I am not playing in campaign mode for the Progression Series.
My stipulation for this build was to use Gandalf but not Elrond/Vilya, and to win by clearing the Balrog with 14 damage from Gandalf after clearing the bridge.
The focus of this deck is to maximize Gandalf's resource expenditure to quickly ramp willpower and combat strength in order to attack the Balrog for 20 on the turn it appears. For example, Gandalf, Beorn and Saruman with a Flame of Anor for 3 already get you to 19. Attachments except for Celebrían's Stone and Hobbit Pipe go on Gandalf, but don't forget you can move the signals away before he takes the plunge.
You have plenty of disposable allies, so don't worry about trying to hold onto Fellowship of the Ring when things get sticky.
The list could be tightened slightly with Daeron's Runes and/or another Self Preservation, although you don't have infinite flexibility to manipulate the top of the deck so it could be risky to run 3 Runes and get stuck with them in hand. Possible cuts are Dúnedain Mark, A Test of Will, and Elrond.